Civilians have been subjected to extended arbitrary detention, disappearances, and even torture by both sides in the conflict in eastern Ukraine, two leading rights watchdogs warn in a joint report.

The July 21 findings by Amnesty International and Human Rights Watch (HRW) accuse Ukrainian authorities and pro-Kyiv paramilitary groups of holding civilians suspected of supporting or having connections with Russia-backed separatists. It says separatists incarcerated civilians suspected of backing or spying for the Ukrainian government.

In some cases, detainees were used as a negotiation chip for prisoner exchanges, the groups say in the report, titled You Don’t Exist: Arbitrary Detentions, Enforced Disappearances, And Torture In Eastern Ukraine.

Authorities in Kyiv reacted to the report by promising to investigate, while a representative for separatists in the eastern regions of Donetsk and Luhansk called the findings "absurd."

'Immediate' Action Needed

The rights groups say they were driven to join forces by the magnitude of the problem.

"The reason why we speak with one voice is because the problem of arbitrary detentions and forced disappearances and torture in connection with the armed conflict in eastern Ukraine is very big and demands immediate action by all sides involved in the conflict," HRW’s Tanya Lokshina told RFE/RL.

The groups looked in detail at 18 alleged cases of arbitrary, prolonged detention of civilians by both sides in the conflict, which has claimed more than 9,400 lives since April 2014.

The report says that, in most cases studied, civilians were held without any contact with the outside world, including with their families or attorneys.

Most of those detained were ill-treated and even tortured, and some were denied medical care for injuries sustained in detention, the report says.

The report says pro-government forces, including volunteer battalions, detained civilians, then handed them over to the SBU, which ultimately moved them into the regular criminal justice system.

WATCH: Ukrainian government forces and Russia-backed rebels have carried out arbitrary detentions and used torture, Tanya Lokshina from Human Rights Watch tells RFE/RL:

Russia-backed separatists, meanwhile, held civilians in isolation for weeks or months without charge and, in most cases, subjected them to ill-treatment, the report notes. In the territories held by the separatists -- parts of the Donetsk and Luhansk provinces -- local security forces operate in a complete rule-of-law vacuum, it says.

Detained, Tortured By Both Sides

One case, that of a 39-year-old man whom the report names only as Vadim, stands out in particular, because it says he was held in secret detention and tortured by both sides.

Vadim was apprehended by Ukrainian forces in April 2015, the report says, while traveling on a bus from Ukrainian-controlled Slavyansk to his hometown of Donetsk -- one of the main cities held by Russia-backed separatists. It says he was questioned by Ukrainian forces about his ties in Slavyansk, called a “separatist thug,” then kept in unacknowledged detention at a base, interrogated and tortured, then transferred to another facility, which the report says was maintained by SBU personnel.

The groups say Vadim spent another six weeks there without any contact with the outside world, was tortured with electric shocks, burned with cigarettes, and beaten by interrogators demanding he admit he had ties with the separatists. After he was released, the report says, he returned to Donetsk only to be immediately arrested by the Russia-backed authorities, who kept him incommunicado for another two months and beat and ill-treated him over suspicions he had been recruited by the SBU during his previous captivity.

WATCH: Town Councilor Tel​ls Of Detention Without Trial By Ukrainian Security Forces

Justice Minister Pavlo Petrenko told RFE/RL’s Ukrainian Service that authorities would seek to find those responsible for any violations.

"It will be possible to conduct adequate checks in cooperation with the [Ukrainian] ombudsman. As the Justice Department we are ready to join to this process," Petrenko said.

Earlier, Oleksandr Tkachuk, the SBU's chief of staff, denied the existence of SBU-run detention facilities. Tkachuk told RFE/RL’s Ukrainian Service that the SBU is ready to provide rights watchdogs access to all its premises to show that it is not holding anyone in arbitrary detention.

But Tkachuk said authorities would study the report and investigate possible human rights violations by members of Ukraine’s law-enforcement agencies.

Representatives of the separatists fighting in eastern Ukraine also denied the existence of secret detention facilities on the territory under their control, parts of the Donetsk and Luhansk regions.

"We have no hidden prisons. I am personally monitoring the treatment of our prisoners and their state," Darya Morozova, who is identified as an ombudsman for the separatist group Donetsk People's Republic, told the Interfax news agency.

Vladislav Deinego, a representative of another group seeking independence from Kyiv, the Luhansk People's Republic, also flatly rejected the findings of the report.

"This is totally absurd. Our security services are obviously engaged in exposing subversive actions and sabotage, but this work is legal and transparent. Any hidden prisons are out of the question," Deinego told Interfax.

Bargaining Chips

The report warns that all people held by the warring sides in eastern Ukraine are protected under international human rights law.

"International human rights law, international humanitarian law ban arbitrary detention, torture, and ill-treatment. The ban on torture is absolute. Torture is, in fact, a war crime," HRW’s Lokshina said.

In some cases, the report says, detentions constituted enforced disappearances, because “the authorities in question refused to acknowledge the detention of the person or refused to provide any information on their whereabouts or fate.”

A particularly serious accusation in the report is that captors on either used the possible release of detainees as a bargaining chip for prisoner exchanges.

"Almost in all of the 18 cases that we investigated for the purposes of our joint report, release of civilian detainees was at some point described by the relevant side in the context of prisoner exchanges," Lokshina said.

In nine out of the 18 cases, they were in fact exchanged, says the document, noting that the practice raises grave suspicions that civilians may be detained intentionally for bargaining purposes.

'Deeply Entrenched Practices'

While it is difficult to estimate the actual number of civilians who have fallen victim to such abuses, the document quotes the UN Office of the High Commissioner for Human Rights (OHCHR), which stated in a report last month that “arbitrary detention, torture, and ill-treatment remain deeply entrenched practices” in the region.

"The cases we documented jointly with Amnesty International may only represent the tip of an iceberg," said HRW’s Lokshina.

The two watchdogs call on both the Ukrainian government and the separatists who hold parts of eastern Ukraine to "immediately to put an end to enforced disappearances and arbitrary and incommunicado detentions."

They call on both sides to apply "zero-tolerance" for torture and ill-treatment of detainees, and urge them to thoroughly investigate accusations of torture and ill-treatment in detention and hold those found responsible to account.